This invention pertains to the field of optical communication systems. More specifically, this invention is related to controlling the polarization of optical signals.
As is known in the art, the performance of optical devices such as tunable filters, optical amplifiers, modulators and receivers are all adversely affected by changes in the polarization of the incident optical signal. For example, in coherent optical receivers, the field intensity of the detected signal is directly proportional to the cosine of the phase difference between the input optical signal and local optical source. Accordingly, when the polarization of the signal and local source are in-phase, the field intensity of the detected signal is at a maximum value. However, when the polarization of the signal and source are orthogonally orientedxe2x80x94i.e., 90 degrees out-of-phase, the detected signal field intensity is reduced to a zero level. Such losses in signal strength are detrimental to the performance of optical systems. Thus, maintaining a fixed and known state of polarization (SOP) of an incident optical signal with respect to the SOP of the local optical source is a problem that must be continuously solved to maintain acceptable performance in optical communication systems.
Methods of matching the SOP between two light sources are known in the art. Polarization controllers are one method used to adjust the polarization of an input optical signal to an arbitrarily set fixed and known output value regardless of the input polarization. An integrated polarization controller using a quarter-wavelength and a half-wavelength plate to provide a fixed polarization output for a varying optical input is disclosed in H. Shimizu, Endless Polarization Controller Using Electro-Optic Waveplates, Electronic Letter, Vol. 24, No. 7, Mar. 31, 1988. A second method of controlling polarization is disclosed in U.S. Pat. No. 5,212,743, Automatic Polarization Controller Having Broadband Reset-Free Operation, to Heismann. Heisman discloses a half wavelength plate (HWP) is interposed between two quarter-wavelength plates (QWP) to provide a fixed and known polarization output for a varying input signal polarization. In this method, the two outer quarter-wavelength plates are synchronized such that there is fixed angular difference between the two outer plates. That is, a change in orientation of one outer QWP causes a corresponding change in the other QWP.
Experimentation has shown that current polarization controllers fail to provide adequate compensation for rapidly changing input polarization. Such failure to compensate for a varying input polarization is typically caused by the controller losing track of the input optical signal. Such loss of tracking lock, known in the art as xe2x80x9ctrapping,xe2x80x9d typically causes a reduction in the signal power of the optical signal in the desired polarization. In cases, the loss in power may exceed 30 percent. As would be understood, such levels of power reduction are detrimental to the performance of optical components that depend upon a fixed and known SOP. For example, in a coherent optical receiver, a loss of 30 percent of the input signal may cause the signal-to-noise ratio of a detected signal to be reduced to such a low level that the optical signal may not be properly decoded.
Further, polarization controllers using an HWP are harder to characterize, contain more distortion, and are less reliable than those using QWPs. This is primarily because the HWP is twice the length of the QWP.
Accordingly, there is a need to provide for improved stabilization of the polarization of optical signals in response to a varying input polarization in order to reduce the number and magnitude of trapping occurrences.
The present invention is an apparatus for improving the control and stabilization of the polarization of an optical signal that varies as the optical signal traverses an optical media such as a fiber-optic cable. By cascading a plurality of quarter-wavelength plate elements and dynamically adjusting the eigenaxis of the quarter-wavelength plate elements in relation to the polarization of a received optical signal, the polarization of a received optical signal may be maintained substantially fixed with fewer occurrences of loss of lock and reduced depth of trapping.
In an exemplary embodiment of the invention a plurality of electro-optical quarter-wavelength plate elements are cascaded together on a low birefringence optical medium. The phase delay introduced by the quarter-wavelength plate elements causes variations in the birefringence of the optical medium that alters the polarization of the incident optical signal. By dynamically adjusting the eigne axis the quarter-wavelength plate elements in response to changes in polarization of an incident optical signal, an arbitrarily set fixed and known polarization of the output optical signal may be maintained.